Piston crown or dish

[GrumpyUnk]

 Trying to figure out which kit to install, I was comparing some 47mm BBKs I have on hand.  Paying attention to how freely the piston moved in the bore when the cylinder was rotated 180, I finally noted the piston tops.
 Some have a small machined area (button) in the center, others have a larger button.  One has no 'button' area at all.  They all are concave when laying a straightedge across from side to side.  It seems the dish is similar, though I did not measure.
 I then took out the latest 39mm piston/cylinder I had replaced.  Its crown(piston) was convex.  The crown had a dome shape to it.
 We all have mentioned increased compression pressure due to using the original stock cylinder head.  I had compared 39mm heads to '47mm heads', and it appeared that the 47's had a machined area around the diameter while the 39's had a flat surface all the way across.
 Considering the use of a 'fire ring' on composition head gaskets, and the burned away steel on an original stamped steel head gasket, I got to wondering how/where the fire ring would meet the combustion chamber when installed.  Using a 39mm flat faced head, the fire ring would be recessed or sandwiched between the bottom of the flat head surface at whatever diameter, and the top of the cylinder wall, again at whatever diameter the fire ring happened to be( I assume(?) at 47mm+xx).
 If a head designated for 47mm use was installed, I have no idea how the gasket would be fitted, i.e., how it would be protected or not from combustion gas and heat.  Given a groove is machined into the bottom of the head surface, exactly where would the fire ring sit.
 I also wondered what the 'dish' in top of the 47mm kit pistons does to compression ratio.  I know it should lower compression, but have not measured the compression result for a 'good' 39mm engine to compare.  For that matter, have not measured those for a 47mm BBK after assembly.  So, there is a recess that lowers peak compression, so it seems the kits were designed for the situation where you do want to limit compression ratio. (Otherwise why cast in a recess and then do some machining to it?)  The 39mm are not machined, at least not to any discernable finish.  The 47mm pistons, all I have seen, have a non-polished surface, appearing to reflect the finish of the casting mold.  I just realized the need/use for the center machined 'button'.  That's where the mold injected/flowed the heated aluminum.  The 'sprue', I think.  The passage for liquid metal would be attached somewhere to the cast piston, and that makes good sense.  Cut it off with a saw, and then polish the surface to eliminate any 'hot spots' on the top of the piston when in service.
 Any thoughts on what the depression does to the CR of these kits?
tom

[bullybike]

nowadays we type TL;DR
or even, a picture is worth 500 words (deflation)

from a jist I would say extra material on a piston cap would be to increase compression ratio. Some generic qmb heads are hemi-or semi hemispheric, others are generally flatter and would have a higher standard CR

Also aftermarket big bore cylindrs seem to have valve half-moons for running cams ground with higher lift.

[GrumpyUnk]

I could type: "nowadays we type TL;DR"


but it would still have no meaning.

Ok. Lazy. Un-read. Fine. At least I looked up the meaning.

[FrankenMech]

The piston compression height and the top shape either domed, flat, or dished would be matched to a specific combustion chamber shape on a specific head to provide the desired compression ratio, squish area, etc. Valve reliefs also figure in to the formula. When rebuilding performance engines we often cc out heads and pistons in order to determine combustion chamber volume and compression ratio. We also check valve clearances. Pistons in larger engines are frequently not round, I don't know how these micro engine pistons are shaped.

Also you are correct, the mold has a sprue where the liquid metal flows into the part. Those sprue marks may be machined away per specs. These little engine parts may be injection molded.

[ThaiGyro]

Grumpy Unk...many words for my eyes to comprehend. However, it appears that you are looking for a good compression or squish band or???

I would prefer to know what you want your engine to do? Wheelie from get-go? Kill cats in the neighborhood? Maybe smoke a few GXSR's?


I prefer to tune the CVT....Viking attitude?

[GrumpyUnk]

This is more a thought process and contemplation on the design. The makers of the 47mm BBK sets refrained from casting the piston crown as a dome(39mm style) and instead designed in a concave 'dish'. I really had not noticed until checking some sets for piston fit. I figured I'd mention it, as it seems everyone and their cousin notes the the CR would be high using a BBK with a stock head. I noted the dish, and how that should decrease the CR from what it would be with a flat or domed piston. IOW, the makers took into account that the market for the 47mm kits was people who were not going to replace the cylinder heads when the installed the replacement BBK. I assume(not living in the FE, but having visited several countries decades ago, that these 139QMB engines/trans are kind of like ants. They are everywhere. Used by families for trips to town, to make deliveries, and to pick up the weeks provisions. They all pile up the miles, and eventually, being a motorcycle engine, air cooled, etc etc, will need some replacement parts to bring back the performance to that of a new machine. Add in a 47mm bump BBK, and you get just a bit more torque to carry the load.
In short, I think the lower end BBKs were built for the home or local market, as a 'rebuild kit in a box' for tired engines. Bigger bore adds a bit of zest to the old buggy. Made it all up, but if the piston crown was indeed flat or domed, it would almost make a new head with larger cc chamber a necessity. Given the concave or dish, one can get a bit of a CR bump, but not obnoxious as the factory starter seems to have no problem cranking over this size, at least.
Some will go as above, TLDR. Fine. I don't care. Their loss, as they don't get to have a possible insight as to how, why, when, where, and so on about products used in their life.(and others' lives also).
In USA, scooters are generally for young persons, perhaps away at university, to get around town, visit friends, and take short jaunts. They are not necessary items as they are in other markets. This is just a possible interpretation of how/why they did what they did when the BBK was designed. That and a nickel.
tom

[Petro]

link

Jun 3, 2020 10:56:01 GMT -5 GrumpyUnk said:

This is more a thought process and contemplation on the design. The makers of the 47mm BBK sets refrained from casting the piston crown as a dome(39mm style) and instead designed in a concave 'dish'. I really had not noticed until checking some sets for piston fit. I figured I'd mention it, as it seems everyone and their cousin notes the the CR would be high using a BBK with a stock head. I noted the dish, and how that should decrease the CR from what it would be with a flat or domed piston. IOW, the makers took into account that the market for the 47mm kits was people who were not going to replace the cylinder heads when the installed the replacement BBK. I assume(not living in the FE, but having visited several countries decades ago, that these 139QMB engines/trans are kind of like ants. They are everywhere. Used by families for trips to town, to make deliveries, and to pick up the weeks provisions. They all pile up the miles, and eventually, being a motorcycle engine, air cooled, etc etc, will need some replacement parts to bring back the performance to that of a new machine. Add in a 47mm bump BBK, and you get just a bit more torque to carry the load.
In short, I think the lower end BBKs were built for the home or local market, as a 'rebuild kit in a box' for tired engines. Bigger bore adds a bit of zest to the old buggy. Made it all up, but if the piston crown was indeed flat or domed, it would almost make a new head with larger cc chamber a necessity. Given the concave or dish, one can get a bit of a CR bump, but not obnoxious as the factory starter seems to have no problem cranking over this size, at least.
Some will go as above, TLDR. Fine. I don't care. Their loss, as they don't get to have a possible insight as to how, why, when, where, and so on about products used in their life.(and others' lives also).
In USA, scooters are generally for young persons, perhaps away at university, to get around town, visit friends, and take short jaunts. They are not necessary items as they are in other markets. This is just a possible interpretation of how/why they did what they did when the BBK was designed. That and a nickel.
tom

Last week I installed an Airsal 50mm. It works great with the stock head and the starter has the power to turn if one doesn't twist the throttle too much.
The shape of the piston top isn't important to me and I see it more as a gimmick in this low output engines...

What concerns me more is the quality of metal, that aluminium really is aluminium and not with added 10-15% tin or so. For example, comparing the Airsal 50mm piston it's 12% lighter than a chinese 47mm I've got laying around despite it's larger...
The Airsal cylinder is all aluminium without the metall sleeve so I can't fairly compare it's weight to a chinese dito. It does show me that the aluminium Airsal using is high quality though...


I see a lot of people bying "BBK 100cc kits" (no, they do not exist) and I am sure they work for a while... My personal experience though tells me that with a good brand many problems described on forums wouldn't be if quality parts where used in the first place... Been there, done that, there is a huge difference.
After wrath and sorrow I five years ago bought a 52.4mm Naraku kit, big valves and all. I should had done that from the start, it just worked and stayed well tuned without hassle. Well, until I gave the bike away to my friends teenager son... He managed to blow the enginge up after 5-6 belts but he had a hard time! 

My 50mm Airsal: www.twostroke.se/airsal-cylinderkit-gy682-cc?gclid=Cj0KCQjw_ez2BRCyARIsAJfg-kuNxGVr0SPnkiafWUNZngWs100cA2dRdGG9GsJj_mv-KtrcKgSk3zsaAspWEALw_wcB